Piston Failures; Pre-Ignition - Mercury MerCruiser MCM 454 Mag MPI Service Manual

454 CID (7.4L) / 502 CID (8.2L)

Piston Failures

Pre-Ignition

Pre-ignition is abnormal fuel ignition, caused by combustion chamber hot spots. Control of
the start of ignition is lost, as combustion pressure rises too early, causing power loss and
rough running. The upward motion on the piston is opposed by the pressure rise. This can
result in extensive damage to the internal parts from the high increase in combustion cham-
ber temperature.
Pre-Ignition Damage
Combustion Differences
PRE-IGNITION CAUSES
1. Hot spots in the combustion chamber from glowing deposits (due in turn to the use of
improper oils and/or fuels).
2. Overheated spark plug electrodes (improper heat range or defective plug).
Index
Page 3A-22
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a
c
a - Ignited By Hot Deposits
b - Regular Ignition Spark
c - Ignites Remaining Fuel
d - Flame Front Collide
SERVICE MANUAL NUMBER 23
b
d
90-861326--1 MARCH 1999
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